One type of powered surgical tool used in orthopedic surgery is the surgical drill. This type of tool includes a housing that contains a motor. A coupling assembly, also part of the drill, releasably holds a drill bit to the motor so that, upon actuation of the motor, the drill bit rotates. As implied by its name, a surgical drill drills bores in the tissue against which the drill bit is applied. One type of surgical procedure in which it is necessary to drill a bore is a trauma procedure to repair a broken bone. In this type of procedure, an elongated rod, sometimes called a nail, is used to hold the fractured sections of the bone together. To hold the nail in place, one or more bores are driven into the bone. These bores are positioned to align with complementary holes formed in the nail. A screw is inserted in each aligned bore and nail hole. The screws hold the nail in the proper position relative to the bone.
In another type of procedure, an implant known as a plate is secured to the outer surfaces of the fractured sections of a bone to hold the sections together. Screws hold the plate to the separate sections of bone. To fit a screw that holds a plate to bone it is necessary to first drill a bore to receive the screw.
As part of a procedure used to drill a screw-receiving bore in a bone, it is desirable to know the end-to-end depth of the bore. This information allows the surgeon to select size of screw that is fitted in the bore hole. If the screw is too short, the screw may not securely hold the nail into which the screw is inserted in place. If the screw is too long, the screw can extend an excessive distance out beyond the bone. If the screw extends an excessive distance beyond the bone, the exposed end of the screw can rub against the surrounding tissue. If this event occurs, the tissue can against which the screw rubs can be damaged.
Accordingly, an integral part of many bone bore-forming procedures is the measuring of the depth of the bore. Currently, this measurement is often taken with a depth gauge separate from the drill. This requires the surgeon to, after withdrawing the drill bit from the bore, insert the depth gauge into the bore. Then, based on tactile feedback, the surgeon sets the gauge so the distal end of the gauge only extends to the far opening of the bore. Once these processes are complete, the surgeon reads the gauge to determine the depth of the bore.
A disadvantage of this protocol is that after the bore is formed, the surgeon must take the time to: insert the depth gauge in the bore; position the gauge properly to ensure the bore depth is accurately measured; read the gauge to determine bore depth; and withdraw the gauge. Having to perform these sub-steps adds to the overall time it takes to perform a surgical procedure. Having to perform these sub-steps thus goes against one of the objective of modern surgical practice; the procedure should be performed as quickly as possible to both minimize the time the interior tissue is exposed to the ambient environment and therefore open to infection and to reduce the exposure of the patient to anesthesia.
To avoid having to spend this extra time measuring bore depth, surgical drills have been proposed that include built in depth gauges. This type drill typically includes a rod that is slidably mounted to the drill housing. The rod is positioned to be parallel with and spaced away from the drill bit. A head is located at the distal end of the rod. The head is positioned to seat around the drill bit. When this drill is used, the drill is positioned so that, while the rod is extended the head is place against the bone around which the bore is to be formed. As the bore is formed, the head and rod remain static. The drill moves towards the head. A sensor mounted to the drill monitors the movement of the drill relative to the rod. The measurement from the sensor of the movement of the drill is employed as the measure of the depth of the bore.
The above type of drill can form a bore in tissue and simultaneously provide a measure of bore depth. A problem with this type of drill is that the rod, which is spaced away from the drill bit by a distance of 0.5 cm or more, and the head, which can have a diameter of 0.8 cm or more, obstruct the surgeon's view of the tissue against which the drill bit is pressed. For this reason, this particular type of drill has not proven to be a popular device for forming a bore while simultaneously providing a measure of bore depth.